1. Field of the Invention
The present invention relates to a filter element which can suitably be used for treatment of an exhaust gas from an internal combustion engine, especially a diesel engine.
2. Description of Related Art
An exhaust gas discharged from an internal combustion engine, especially a diesel engine, includes a particulate matter (PM) and ash. To remove the PM and ash from the exhaust gas of the diesel engine or the like, a diesel particulate filter (DPF) is used. The DPF is a filter having a honeycomb structure constituted of a large number of mutually parallel fluid through channels partitioned by porous partition walls. Adjacent inlet and outlet portions of the fluid through channels are alternately plugged, and the partition walls of the fluid through channels are used as filter surfaces of the exhaust gas. When the DPF is used for a long period of time, a predetermined amount or more of the PM and ash is deposited on an inner part of the filter, so that a pressure loss increases, an ability to remove the PM and ash deteriorates, and a function of the filter cannot be exerted. Therefore, it becomes necessary to perform a regeneration treatment of the DPF. The PM deposited on the DPF can be removed by heating/burning at regeneration, but the ash cannot be removed by the burning at the regeneration, and substantially has to be deposited on inlet cells of the DPF.
The ash is deposited on the DPF mainly in order from terminal ends of the inlet cells to an inlet side of the DPF in a cell direction, but as an amount of the ash to be deposited increases, a filter area substantially decreases, which results in rapid increase of the pressure loss. To avoid the increase of the pressure loss, it has been suggested that an open area of each of the inlet cells is increased as compared with outlet cells, or a plugging pattern is contrived to increase the number of the inlet cells, thereby increasing a capacity of each inlet cell, so that a larger amount of ash can be deposited (see Patent Documents 1 and 2).
On the other hand, for the purpose of removing the deposited PM to promote the regeneration of the DPF, the DPF is usually coated with an oxidation catalyst. When each cell is formed into an obtuse cross-sectional shape such as a hexagonal cross-sectional shape as in technologies disclosed in Patent Documents 2 and 3, an inner portion of the cell can be evenly coated with the catalyst. In consequence, an expensive catalyst containing a large amount of noble metal can be saved, as compared with cells each having an intersecting point of 90 ° or less, for example, quadrangular cells in which the catalyst is unevenly deposited on each intersecting portion with each partition wall and in which a large amount of substantially unused catalyst remains.
Moreover, in FIG. 11 of Patent Document 2, there is disclosed a honeycomb filter for the purpose of achieving both of enhancement of catalyst coat properties of the cells each having the hexagonal cross-sectional shape and increase of a capacity for the ash to be deposited. That is, according to the honeycomb filter disclosed in FIG. 11 of Patent Document 2, six inlet cells are provided around one outlet cell, to set a total capacity of the inlet cells of a honeycomb carrier to be twice as much as that of the outlet cells, thereby increasing the capacity for the ash to be deposited.
However, in Patent Document 2, partition walls shared by the inlet cells are present, and hence the number of partition walls which effectively collect the PM and the ash substantially decreases. That is, there has been the problem that an effective GSA (a value obtained by dividing, by a volume of a carrier, a total surface area of the partition walls shared by the inlet cells and outlet cells) decreases and a pressure loss increases.
Furthermore, according to Patent Documents 3 and 4, to increase an effective GSA, inlet cells are deformed. That is, there is disclosed a filter structure where a length of a partition wall shared by inlet cells is shortened to increase the effective GSA.
However, in the filter structures disclosed in Patent Documents 3 and 4, the increase of the effective GSA results in decrease of a total volume of the inlet cells, which has caused the problem that a capacity for ash to be deposited decreases as compared with the technology of Patent Document 2.
[Patent Document 1]JP-2005-270969-A
[Patent Document 2]JP-03-49608-B
[Patent Document 3]JP-2009-537741-T
[Patent Document 4]JP-2011-509816-T